In magnetic data storage devices, data information is stored and retrieved through the use of a magnetic transducer or head performing read and write operations on a magnetic medium. The transducer is also commonly referred to as a head. The magnetic head is deposited on a slider and positioned close to the surface of a disk where the magnetic materials are disposed forming the magnetic disk medium. The gap between the magnetic head and the surface of the magnetic disk medium is known as head-medium separation or magnetic separation. When the magnetic disk spins about the spindle on which it is mounted, the magnetic head on the slider ‘flies’ above the surface of the magnetic disk. The slider is able to ‘fly’ as the air between the slider and the disk surface supports the slider. The spacing between the air bearing surface of slider at the trailing edge and the surface of magnetic disk is defined as the flying height or the physical spacing. The increase in demand for higher data storage capacity has lead to an increase in data track density and data linear density on a magnetic (disk) medium. To achieve higher linear density, and to compensate readback signal loss in magnetic separation, a head is required to fly progressively closer to the disk to maintain signal strength. The physical spacing is currently around 10 nm. With the annual growth rate of areal density at 60–120%, there is a need for the physical spacing to be reduced to less than 10 nm. For example, at an areal density of 200 Gb/in2, a stable physical spacing of 5 nm–7 nm would be required. If the areal density is increased further, the head-medium separation will need to be reduced further to ensure adequate signal strength between the magnetic heads and the data track. While the physical spacing is reduced, the magnetic head should not contact the disk surface as this would degrade the performance of head disk interface and may even damage the data recorded in the magnetic disk medium. Therefore, the stability of the head disk interface needs to be constantly monitored and maintained to ensure that the transducer is sufficiently close to the surface of the disk medium for adequate signal strength without risk of the head “crashing” onto the disk medium.
A conventional method for determining the flying height is the optical testing method which is based on optical interference principles for determining the slider-disk interface. The use of a special transparent glass disk in this method for testing the flying height does not reflect the actual situation of the real head disk interface (HDI) with the lubricant and the topographic features. Another limitation is the limit in optical wavelength with a resolution at around 0.25 nm, which is insufficient to accurately provide the physical spacing measurement in the sub-10 nm range. This method measures the physical spacing at head gimble assembly (HGA) level but not during head stack assembly (HSA) level or during drive level manufacturing. Hence this method cannot facilitate quality control during the manufacturing process of disk drives. Therefore alternative methods were developed so as to enable head-medium spacing measurement during the manufacturing process.
Alternative methods are based on the Wallace equation where data information encoded on the magnetic medium is retrieved as readback signals, which is used to measure the head-medium spacing. In these prior art methods, the spacing to be measured is suitable for estimating the relative change in head-medium spacing but not the absolute head-medium spacing.
U.S. Pat. No. 4,777,544 demonstrates a typical harmonic method where the measurement of the flying height is based on relative spacing changes. The amplitude of the readback signal is detected at different frequencies and a ratio of two of such amplitudes is computed which forms mainly as a function of the head-medium spacing. While interfering factors like the effect of the track width and the remnant-moment-thickness (MrT) are eliminated by taking the ratio of the amplitudes to obtain a more accurate measure of the head-medium spacing. The absolute head medium spacing is measured by having the head contact the disk surface, which may cause damage to the giant magnetoresistive (GMR) head.
Therefore, it is desired to have a method that would address the absolute head medium spacing measurement without the contact of the head and medium during the measurement.